Just last week, a teacher visited my office with a pink and itchy eye. Pink eye, or conjunctivitis, was spreading through her elementary school classroom. This is one of the most common, contagious, and (thankfully) treatable eye conditions. There are roughly three million cases in the United States every year.

The “-itis” in conjunctivitis refers to inflammation, in this case of the conjunctiva, the thin, clear tissue that lines the inside of the eyelid. The inflammation makes the blood vessels of the eye more visible, giving the eye a pink appearance.

Pink eye is diagnosed with an eye exam, and your doctor may take a culture of the fluid in the eye if it is a severe case. There are three types of conjunctivitis:

Bacterial conjunctivitis results in a sticky, yellow or green eye discharge. Upon awakening, the eye may feel as if it is glued shut. Bacterial conjunctivitis is easily spread by direct contact by hands that have touched the eye. It’s caused by bacteria, including staphylococcus aureus, streptococcus pneumonia, or haemophilus. It tends to occur in one eye, and may accompany an ear infection.

Bacterial conjunctivitis typically needs treatment. Mild bacterial conjunctivitis may get better without antibiotics, but antibiotic eye drops or ointment can help shorten its course, reduce its spread to others, and lessen the risk of complications.

Viral conjunctivitis causes itching in the eye, a watery discharge, and sensitivity to light. It is highly contagious and can be spread by coughing and sneezing. It is usually caused by the adenovirus, and might accompany an upper respiratory tract infection, cold, or sore throat.Most pink eye cases caused by viruses are mild and will resolve by themselves in a week or two without treatment or lab tests. Antiviral medication may be prescribed for serious cases, including infections caused by herpes simplex, varicella zoster (chicken pox and shingles), or rubeola (measles).

Allergic conjunctivitis symptoms include watery, itchy eyes, a runny nose, sneezing, and scratchy throat. It affects both eyes and it is not contagious. It is caused by allergies to pollen, dust mites, molds, animal dander, irritants including contact lenses and solutions, chlorine from a swimming pool, smog, or makeup. It often occurs seasonally, during high pollen counts of the spring, or mold in the fall.

Most cases of allergic conjunctivitis are mild and will resolve on their own. Symptoms can be treated with artificial tears for the dryness, and cold packs for the inflammation. Eye drops and oral medication for allergies may help. More importantly, reduce contact with the allergen (such as pollen, or animal dander).

See your doctor if your pink eye is accompanied by moderate or severe pain or by changes in vision (including sensitivity to light, or blurring), if the eye is intensely red, if the pink eye gets worse, or if it simply doesn’t go away.

How can we prevent conjunctivitis? As my teacher patient learned the hard way, viral and bacterial conjunctivitis are highly contagious and spread easily! Take these preventive steps:

■ Wash your hands often with soap and warm water or an alcohol-based hand sanitizer, especially if you have touched an infected person’s eyes, bedding, or clothing.

■ Avoid touching or rubbing your eyes.

■ If you have conjunctivitis, gently wash any discharge from the eye several times each day.

■ If one eye is infected, don’t use the same eye drop dispenser or bottle for the non-infected eye.

■ Don’t share makeup, makeup brushes, contact lenses or solutions, or eyeglasses with anyone who is infected. To avoid re-infecting yourself, throw away or sterilize any of these items you may have used while infected.

Dr. Alan Frischer is former chief of staff and former chief of medicine at Downey Regional Medical Center. Write to him in care of this newspaper at 8301 E. Florence Ave., Suite 100, Downey, CA 90240.